Relays for connection and disconnection of inductive, capacitive and resistive loads, and their combinations are well known. Such relays are disclosed in U.S. Pat. No. 5,309,068, entitled TWO RELAY SWITCHING CIRCUIT FOR FLUORESCENT LIGHTING CONTROLLER, in the name of Simo P. Hakkarainen et al, and assigned to the assignee of the present invention; and are also described in U.S. Pat. No. 4,855,612, U.S. Pat. No. 4,772,809, U.S. Pat. No. 4,209,814 and U.S. Pat. No. 3,864,604.
In the prior art, it was known to employ a main switching relay contact with a parallel circuit consisting of a controllably conductive device such as a triac, a pair of antiparallel connected SCRs, a MOSFET, an IGBT or a thyristor type device in series with an auxiliary switching relay contact. In operation, and during closing, the auxiliary relay contact is first closed, and the thyristor is next turned on, creating a conductive path in parallel to the main relay contact. The main contact is then closed with little or no arcing.
In U.S. Pat. No. 5,309,068, the series path containing the auxiliary relay contact and thyristor was then made nonconductive until the next main relay contact closing operation. The main load circuit is opened simply by opening the main contact.
In U.S. Pat. No. 4,772,809, to close the circuit, the series circuit of auxiliary relay contact and thyristor is first made conductive and the main relay contact is closed without arcing. However, the auxiliary relay contact remains closed so that, when opening the load circuit, the main contact is opened, and current can commutate into the closed series circuit. Thus, the main relay contact opens with little or no arcing.
It has been found that circuits of this type, in which the auxiliary relay contact remains closed while the main relay contact is closed, are subject to damage or destruction due to surge currents created, for example, by a lightning strike since the extremely high surge current, which is partly diverted into the closed series circuit, can be sufficiently high to damage or destroy the thyristor. However, capacitive inrush current during closing of the main relay contact on a capacitive load can damage the main contact if it is closed too soon after the closing of the series circuit. If, however, the delay in closing the main switch is too long, the total energy applied to the triac will cause it to heat to the point where it requires a heat sink.
These requirements are interrelated and have created numerous problems in the design of these circuits.